Three general types of phosphate deposits exist--igneous, sedimentary, and metamorphic. All three types supply significant quantities of phosphate to world markets, but sedimentary deposits are by far the most important. Most of the world's phosphate production is used for fertilizer products.
The predominant phosphate mineral in phosphate ore is apatite. The chemical formula of apatite is often written as Ca.sub.5 (PO.sub.4).sub.3 F, but considerable departure from this formula is possible. Substitution of chloride and hydroxide ions for fluoride ion, carbonate substitution for phosphate and magnesium or sodium substitution for calcium are but a few of the many possible ion substitutions (Lehr, J. R., Proceedings, Fertilizer Industry Roundtable, 61, 1967).
The gangue constituents most commonly associated with unaltered sedimentary phosphate rock deposits are quartz, clay minerals, feldspar, metal sulfides, organic matter, calcite and dolomite. Reworking the deposit by stream activity, ground water leaching and weathering enriches the deposit by removing clays and carbonates. The accessory minerals in these weathered and reworked deposits are usually feldspar, quartz, some clay, metal oxides, and gypsum.
Phosphate rock producers most actively seek the reworked and weathered deposits because these ore bodies contain less clay, and magnesium and calcium carbonates. Producers remove accessory minerals from apatite by a combination of desliming and sizing followed, in some cases, by fatty acid and amine flotation.
Phosphate ores containing silica-based accessory minerals are particularly amenable to this type of beneficiation [Beall, J. V., Min. Eng. 80-114 (1966); Smani, et al, Trans. Soc. Min. Eng. AIME, 258 168-182 (1975)] because there is sufficient difference in surface properties between silicate and phosphate minerals to allow easy separation by flotation. But in the United States, particularly Florida, these high grade weathered deposits are becoming depleted and processors must turn to the more abundant but lower quality deposits.
The lower quality deposits contain higher amounts of iron, aluminum and magnesium impurities. Because separation of these impurities from apatite is never complete, the use of lower quality phosphate deposits will result in lower quality phosphate rock.
In the United States, most phosphate rock is converted to fertilizer products by first dissolving the rock in sulfuric acid. Major products of this reaction are gypsum and phosphoric acid, the latter containing about 28 to 30 percent P.sub.2 O.sub.5. Any iron, aluminum and magnesium in the phosphate rock also dissolves in the acid. Gypsum is discarded and the dilute phosphoric acid is further concentrated to 40 to 54 percent P.sub.2 O.sub.5. The concentrated acid is either ammoniated to ammonium phosphate or converted to triple superphosphate by reacting the concentrated acid with more phosphate rock or converted to solutions of ammonium polyphosphate. Concentrating phosphoric acid is the most energy intensive and hence one of the most expensive steps in the conversion of phosphate rock to fertilizer.
Because iron, aluminum and magnesium in the rock report to the phosphoric acid, the consequences of declining rock quality to the fertilizer industry are important. Although these impurities are initially soluble in phosphoric acid, they subsequently precipitate and thereby cause serious problems to fertilizer producers. For example, an excessive amount of sludge, principally complex iron and aluminum phosphates, may form in phosphoric acid made from low-quality phosphate rock. The sludge may form when it is first concentrated or it may later form when the acid is stored before use. When formed during concentration, the sludge fouls the heat exchangers of the acid concentrator causing excessive downtime and cleaning expense. When formed in the concentrated acid, it fills the acid storage tank and stops up pipes and valves.
All of the iron, aluminum, and magnesium in the acid does not precipitate in the sludge. If an excessive amount of these impurities in the acid reach the finished fertilizer products, further problems arise. For example, it may not be possible to reach industry accepted nitrogen grades in diammonium phosphate because cationic impurities--iron, aluminum and magnesium--displace ammonia in the diammonium phosphate product. If triple superphosphate (TSP) is made from acid containing excessive magnesium, the TSP may be hygroscopic.
Thus, there is a need to purify phosphoric acid before concentration and conversion to finished fertilizers. Since concentrating phosphoric acid is energy intensive, and hence costly, it would also be desirable to convert the purified dilute phosphoric acid directly to finished fertilizer, thereby eliminating this costly concentration step.